Method of chromium plating



Dec. 8, 1936. R. SCHNEIDEWIND 2,063,197

METHOD OF CHROMIUM PLATING Filed Aug. 30, 1933 F RELATIVE THROWING PowER J EFFECTorsome ACID ON CHRONIC ACLU PLATING BATHS =|o0 TEMP-45 c CF05,

RELATI VETHROWINQ PDWER I00 500 RATIO 61 0 some Amy/y (M h (I Q Q o PERGENTINCREASEIN TH Rowmq POWER EME ZU Tab/lard Sc//readem'nd Patented Dec. 8,1936

UNITED STATES METHOD OF CHROMIUM PLATING Richard Schneidewind, A1111 Arbor, Mich, as-

signor of one-tenth to the Government of the United States of America, represented by the Secretary of the Navy Application August 30, 1933, Serial No..687,414

10 Claims.

This invention relates to a method of chromium plating and more particularly to a method of increasing the throwing power and general usefulness of a chromium plating bath by the addition thereto of a considerable excess of a buffer or stabilizing agent over the proportion of such agents previously used in chromium plating baths. 7

It has heretofore been suggested (British Patent- No. 254,757) to employ from 1 to 12 grams per liter of boric acid in a bath comprising about 5 200 grams per liter of chromic acid and from 1 to 12 grams per liter of chromium sulphate (C12(S04)3H2O). It has also been proposed (Suzuki U. S. Patent No. 1,600,076) to employ a solution consisting of from 5 to of chromic acid, from 5 to 15% of chromium sulphate and 5% of boric acid. A solution, however, with such a high sulphate content as Suzuki discloses is not operative to produce satisfactory chromium plated articles.

I have now found that a remarkable and un-' expected increase in the throwing power of a chromium plating bath can be efifected by the use of bufier or stabilizing agents in such proportions} as to provide a pH in the cathode film below, but not too far below the precipitating point of chromium oxides or hydroxides. ,My theory is that if the pH value is too low or too high, a satisfactory chromium plate cannot be obtained.

.p'I'he present invention contemplates the use, for

instance, of boric acid in amounts equal to or in excess of grams per liter of plating solutions. Below about 20 grams per liter,of boric acid, the

' addition of boric acid to the usual chromium plating solution containing around 250 grams per liter of chromic acid and about2 /g grams of sul phate radicals has practically no efiect upon the throwing power of the solution. At about 20 grams per liter of boric acid, however, there is an abrupt change in the throwing power of the solution and the throwing power increases with the increased percentages of boric acid up to about 50 grams per liter of boric acid and remains substantially constant with further additions of boric acid up to the saturation point for the solubility of boric acid.

The advantages of an increased throwing .power are that it enables either a more satisfactory plating of irregular and complex articles, or effects a reduction in the plating. time, or a combination of thesev two. Y I

As is well known, the throwing ,power of chromium plating solutions heretofore has been notably poor in comparison with the throwing power of plating solutions of other metals. Considerable difficulty has been experienced in the past in plating articles of an irregular surface, since chromium will tend to plate much more readily on the projections than on the hollows or recesses ofsuch articles.

Although it has long been known, as indicated above, that boric acid could be used in minor proportions in chromium plating baths, it was apparently not recognized that by usinglarger proportions of boric acid, with proper.

tion to provide a method of chromium plating that is of greater general usefulness and which is less sensitive to fluctuations in the proportions of sulphate radicals present in the bath.

It is a further important object of this invention, to provide a chromium plating bath containing a relatively large proportion of a proper or suitable buffer or stabilizing agent, whereby the hydrogen ion concentration, or pH value, at the cathode film may be stabilized within the proper range.

It is a further important object of this invention to provide a chromium plating bath and method of operation capable, of modifying the crystalline characteristics of the deposit of chromium and adapted to prevent the deposition ing description and appended claims.

' On the drawing:

Figure 1 is a chart showing the-eifect upon the relative throwing power of the bathof varying concentrations of boric acid in chromium plating baths containing varying quantities of CrOa.

Figure 2 is a chart showing the relations between the concentration of boric acid in a chromium plating bath containing about 250 grams per liter of. C103 and the throwing'power of the bath for various ratios of CrOa to S04.

The chromium plating baths now in general commercial use are those having approximately the same formula as the so-called Sargentsolution, namely, about 250 grams'perliter oi chromic acid (C103) and about 2.5 grams per liter of sulfate radicals (S04), introduced either as chromium sulfate or as a free sulfuric acid.

, In chromium plating baths of this type, it -is invention will become apparent from the followgenerally conceded that the ratio between CIO: and S04 radicals should be about 100 to 1 and in general'not less than about 50 to 1 or more than about 250 to 1. When the ratio of chromic acid to sulfate radical lies outside of these ranges, the throwing power of the bath is decidedly poor and it is diiilcult, and frequently impossible, to obtain a bright deposit.

I have now found that by using a relatively large proportion of a buffer, such as boric acid or phthalic acid, or salts containing these acid radicals, the ratio of chromic acid to sulfate radical need not be so closely controlled,'while at the same time the throwing power of thebath is greatly increased. Boric acid, up to about 20 gr. of boric acid per liter of the plating bath has very little effect on the throwing power of the bath, but with concentrations of boric acid above 20 gr. per liter, a continuous increase in the throwing power of the bath with increasing proportions of boric acid is noted up to about 50 gr. per liter concentration. When the concentration of boric acid goes above about 50 gr. per liter,

- there is usually only a very slight further increase, if any, in the throwing power of the bath. Therefore, although no detrimental effect upon the bath is experienced from the use of'concentrations of boric acid above about 50 gr. per liter, 1. e., up to the saturation point of boric acid in the particular bath and at the temperature employed, it is more economical not to exceed 50 gr. of boric acid per liter of plating bath.

In general, the increase in throwing power with the use of from 20 up to 50 gr. of boric acid per liter holds true for varying concentrations of chromic acid in the plating bath, althoughthe eifect is greater with some concentrations than with others. This is best shown by reference to Fig. 1 of the drawing, wherein the relative throwing power of a chromium plating bath forfvarious concentrations of boric acid and chromic acid is shown. If 250 gr. per liter of chromic acid be taken as a typical concentration of a commercial-plating bath, it will be seen that the throwing power of the bath is practically constant up to about 20 gr. per liter of boric acid and then increases rapidly up to about 50 gr. per liter of boric acid. In general, the limits for concentrations of chromic acid in commercial plating baths lie between about 100 gr. per liter as a minimum concentration and about 450 gr.

whereas above the upper limit of the plating range, the deposit is-burned.

In order to determine the effect uponthe plating range of-the use of boric acid in a chromium plating bath, test strips were electrolyzed in the.

respective baths at various temperatures and at various current densities. The strips used were copper strips 35" wide and 3" long, and these were suspended centrally of-a circular anode about 3" in diameter. A voltage of between 3 and 3.8' preferably just over- 3, was used and the time of plating was that used in commercial stabilize the bath.

practice, viz. 2 minutes at high current densities and 3 minutes at low current densities.

In the table below, the so-called standard bath is a bath containing 250 gr. per liter of chromic acid and 2.2 gr. per liter of sulfate, whereas the so-called stabilized bath of my invention is a 'bath containing 250 gr. per liter of chromic acid, 2.2 gr. per liter of sulfate and 50 gr. per liter of boric acid.

The plating range for bright'chromium plate, using these baths, is as follows:

A measure of the throwing poweris obtained .by dividing the upper limiting current density by is applied to the recess portions without burning.

The following table gives the results of calculating the relative throwing powers in this manner:

Percent Temp.- Standard Stabilized 1'. bath bath g2? 11' a. s 5.0 42.8 113 a. 4 0.0 7a. s 140 0.0 10.0 00.7

variations in the ratio of chromic acid to sul-- fate radical in the bath, as well as with variations in the percentages of boric acid in the bath. This is illustrated in Fig. 2, wherein the concentration of boric acid in grams per liter is plotted against the percent increase in throwing power for diflerent ratios of CrO: to S04. The numbers at the ends of curves indicate the ratio of GrOa to S04. From a consideration of Fig. 2, it is apparent that there is a fairly proportional increase in the throwing power for an increase in the concentration of boric acid up to about 50 grams per liter and that below a concentration of boric acid of about 20 grams per liter, the increase in throwing power is relatively slight.

j The curves of Fig. 2 also establish that where the S04 radical contentis low, .less boric acid is required, whereas when the S04 radical content is high, more boric acid is necessary in order to For satisfactory plating results, I find that it is possible to use concentrations of chromic acid of between and 450 grams per liter with a ratio of chromic acid to sulfate radical of between 40 to 1 and 500 to 1,- provided that boric acid is added to the bath in quantities of from 20 grams plating acce e? per liter up to saturation. Withbaths having compositions within this general range, bright plating can be obtained provided that the temperature is properly controlled. The effect of variations in the ratio of CrO: to $04, at various temperature ranges, upon the throwing power is shown in the following table:

Current density for bright plate, amp/ft? Ratio Standard 113th Stabilized bath hrow- Throw- 80 Upper Lower T in Upper Lower g mg 8/1 804 limit limit power. 111311; 111111; power AT 2" C. OR 77 F.

AT 45 C OR 113 F AT 60 C. 0R 140 F It is readily evident from the foregoing table .that plating solutions containing 50 grams per liter of boric acid have much better throwing power and will permit greater variation in sulfate content without detriment to the result. It is especially interesting that no plate is obtainable in the total absence of sulfate; thus it is evident that the borate radical is in no sense an equivalent of the sulfate' radical, perhaps because the sulfate ion enters into a complex cation, whereas the stabilizing or bulfer borate radical does not. However,,the comparatively good results may be obtained with the use of much less sulfate where an excess of boric acid is used than in the absence of boric acid.

The chromium plating baths to which my invention applies, are, in general, baths containing from 100 to 450 gr. per liter of chromic acid (CI'Oa), and preferably about 250 gr. per liter.

While the ratio of chromic acid to sulfate radical in these baths may vary between 40 to l and 590 to 1, I prefer a narrower range of between 100'to l and400 to 1. Also, while quantities of boricacid from 20 gr. per liter up to the saturation being able to plate irregular articles having projections and hollows, or recesses, of considerable magnitude'is very great.

Although this invention has been described specifically in connection with the use of boric acid, it will be understood that other buffers, such as phthalic acid or phthalic anhydride, may be ued in place of boric acid and in equivalent pro-* portions depending upon theirsolubllity. Both boric acid and phthalic acid, or salts containing the respective acid radicals, stabilize the acidity,

, or pH value, at the cathode and to this phenomenon I attribute many of the advantages flowing from the use of '-boric acid or phthalic acid in chromium plating baths.

When using phthalic acid, I have found that.

copper and nickel, and then plated with chro- I mium in baths such as herein described. Also, as is customary, a lead anode may be conveniently used in the plating operation. A temperature of about 45 centigrade ispreferable, although temperatures of between 25 and 60 centigrade may be used, under some circumstances. Also,

current densities of approximately 100 amperes per square foot and the voltages corresponding with present chromium plating practices should preferably be used.

I am aware that numerous details of the process may be varied through a wide-range without departing from the principles of this invention,

and I, therefore, do not purpose limiting the patent granted hereon otherwise than necessitated by the prior art.

I claim as my invention:

1; The method of increasing the throwing power of chromium plating baths containing from 100 to 450 grams per liter of CIOs and from 0.5 to 6.2 grams per liter of $04, which comprises incorporating buffer radicals into said baths in amounts equivalent to from 20 grams of boric acid per liter up to the saturation point for boric acid under the conditions obtaining.

2. The method of increasing the throwing power of chromium plating baths containing from 100 to 450 grams per liter of CrOa and from 0.5 to 6.2 grams per liter of S04 which comprises incorporating borate radicals into said baths in amounts equivalent to from 20 grams of boric acid per liter up to the saturation point for boric acid under the conditions obtaining.

3. The method of electrodepositing' chromium from solutions containing from-100 to. 450 grams per liter of chromic acid and sulfate radicals in the proportions of from 40 to 500 parts of CrOa to 1 part of S04, which comprises reducing chromic acid at the cathode in the presence of from 20 grams per liter of boric acid up to the saturation point for the solubility of boric acid in said solution.

4. The method of electrodepositing chromium, which comprises reducing chromic acid at the cathode at a temperature of between 25 and 60 CrO 100 to 450 so. 0.5 to 6.2

H3303 20 to saturation 75 the ratio of CrOa to $04 in said solution being between the limits of to 1 and 500 to 1.

5. The method of electrodepositing chromium from solutions of chromic acid, which comprises reducing the chromic acid at the cathode in the presence of about 135 to 350 grams per liter of chromic acid, 20 to 60 grams per liter of boric acid and a quantity of sulfate in the proportion of about 1 gram of sulfate to 100 to 400 grams of chromic acid in solution, while maintaining a current density of about 100 amperes per square foot and a temperature'of about C. in the solution.

6. The method of electrodepositing chromium from solutions of chromic acid, which comprises reducing the chromic acid at the cathode from a solution 'having approximately the following composition expressed in grams per liter of solution:

CrOz 250 S04 I 2.2 HaBO J while maintaining a current density of about amperes per square foot and a temperature of about 45 C. in the solution.

7. The method of increasing the throwing power of chromium plating baths containing less than 450 grams per liter of CrOa, which comprises reducing the chromic acid at the cathode in the presence of not less than 20 grams per liter of boric acid.

8. The method of. increasing the-usefulness of a chromium plating bath consisting of a solution of chromic acid and sulfate radicals as the essential ingredients, which comprises incorporating into said solution an amount 0! boric acid equal to from 20 grams per liter to that required to saturate said solution under the conditions of plating obtaining.

9. The method of increasing the throwing I proximating saturation.

RICHARD SCHNEIDEWIND. 

